I think the bigger rims are visually appealing, but when I speak to guys with bigger rims seems like a maintenance headache. One guy I know has a 330xi, and he just had to have all four of his prized HREs re-chromed. He had to actually send them in and paid a premium on the shipping.
The more pertinent issue is this: I do as much as I can to make my X5 look and drive awesome, and I want to keep at it, or else I wouldn't be on this forum. But it's also my work-commute vehicle and I'm putting 80 miles on it EVERY day. I would say I'm on pace for 24,000 miles/per year. Most of that is NJ Turnpike and Garden State Parkway rush hour traffic.
For those of you not familiar with NY/NJ roads: potholes, road debris, flying pebbles, and gravel kicked up by 18-wheelers are an ever-present fact of life. Luckily, my OEM 18s have thick, beefy Michelin Energy MXV4s, and they have held up to the most abusive roads. No flats yet (I know I'll regret saying that

). I'm worried that with my commute, if I switch to a nice set of 20s, my life will become an adventure filled with pothole-bent rims, flat tires (due to their lower profile?), and more rattle-and-roll (because of decreased tire cushioning?)
Are these just mis-conceptions on my part? BTW, I'm attaching a pic of my current setup. I don't think the OEM rims look too bad, but I always wonder "What if...."